Rhododendrons

Volunteer, 3hrs/wk, Jenkins Estate Garden, sponsored by the Tualatin Valley Chapter of the American Rhododendron Society, and Tualatin Hills Parks and Recreation District in Aloha, Oregon.

Hybridizer. See examples of my hybids at about 7 months, 18 months,and 3 years. Serious home gardeners can see the mini-greenhouses I use, basically made from bent 10' sections of electrical conduit. This picture shows them in the summer configuration, covered with 50% shade cloth. It also shows a mini-lathhouse from 3 sheets of commercial cedar lattice. This picture shows them with clear plastic ends attached, ready for colder weather. The milk jugs are filled with water as my attempt to increase "thermal mass". This picture shows them on a clear and frosty but calm morning with the transparent plastic cover unrolled over the top, but the "shirt-tails" sticking out. During a snowstorm I can tuck in the "shirt-tails", and button them up snug. During summer I have light frames covered with 50% shade cloth for plants in full sun. The legs are adjustable for height, and when the weather changes they are easily disassembled.

Rhododendrons are wonderful plants. They are glorious bloomers. Relatively disease free. And relatively low maintenance. But there are a few things gardeners should know.

Things every gardener should know about Rhododendrons.

Above all, rhododendrons are very shallow rooted. The roots form a dense fibrous mat under the dripline. This one fact affects most aspects of gardening with rhododendrons.

• Rhododendrons are relatively easy to move. Even large plants, that may have outgrown a previously suitable location, will rarely have roots going down deeper than one shovel depth.
• Old, well established rhodys may seem to have a hard "cap" covering the soil under them. This is the top of the root-ball. Do not cultivate under rhodys. Rhododendrons benefit from being well mulched. It cuts down on evaporation of soil moisture, and cools the soil temperature.
• Rhododendrons need water. When the surface layers dry out during summer they need supplemental watering.
• And a "good soaking" isn't going to help. Moisture more than 6" or 8" down is out of reach.
• Rhododendrons' fine roots can be susceptible to root rot. They need good drainage.
• Rhododendrons have trouble with soggy conditions. They don't have roots that reach beyond the immediate locale of the plant to better conditions.

What's the difference between a rhododendron and an azalea?

In short, not much. Both are members of the Genus Rhododendron. The taxonomists have a set of rules, that sometimes change, for dividing them. You can just consider there are a couple different species that are called rhododendrons, and a couple that are called azaleas.

If you want a simple, generally reliable, way to tell when you're looking at one, azaleas have one stamen per petal lobe, always five lobes. Rhododendrons have two stamens per petal lobe, and some have more than five lobes.

Maintenance

Immediately after blooming rhododendrons should be dead-headed, the spent flower trusses should be picked or cut off. This one thing will do the most to keep your rhododendrons growing and blooming at their best.

Here is a good example of the "truss" structure on which most rhododendrons bloom. Note the brown, scaley section where truss attaches to the branch. This is generally a weak spot there the spent truss can be snapped off. I prefer to use a small, needle-nosed pruner to make a clean cut.

It's a good idea to wear gloves—they produce a sticky substance that is difficult to remove. Neither soap and water, nor many hand cleaners, seems to work very well. (WD-40 dissolves it well enough, but I prefer not to wash my hands with it.) Helen tells me mineral oil works with a little effort.

Immediately after blooming is when rhododendrons should be pruned. Otherwise you may cut off next year's bloom. But some varieties bud freely and others hardly at all. Have a care until you know which yours is.
 

Usually about the time of blooming and pruning, when the new growth is in the "candle" stage you should pinch off many of the single candles to promote branching, if your rhododendron buds readily enough. Sometimes two candles develop more or less equally. That's a good thing. There's a balance to be struck here: some rhodys won't flower as readily on the resulting branches, some do, some develop two or more buds readily on their own, some don't. So pinch enough to keep the plant full and well branched, but no more than necessary.

Here is an example of "Avalanche" showing a flower bud soon to pop, and a young growth bud not quite to candle stage yet. If left it would have elongated, burst out with leaves, then the branch would have extended. The vestigial buds would have remained undeveloped. Snapped off, the vestigial buds will begin to grow, developing two or more new branches. (We hope.)

If you don't get to them soon enough, and the leaves begin to come out, I wouldn't pinch them. The vestigial buds may not develop fully then. Below you can see the result of pinching last year.

Several weeks later we see there are two new buds growing rapidly. This time they stemmed from vestigial buds right under the original. The remaining vestigial buds in the lower leaves are still undeveloped. They are still useful. Suppose we hadn't pinched out the original bud, and two or three years in the future of single buds extending we decide to restructure the branch. This joint, or close to it, would be a candidate pruning point. That vestigial bud at the base of the leaf will be a good candidate for development. The older they get the less likely, but it's there and it's nice and plump. On some you wouldn't even see a vestigial bud like this. I might take the chance.
 

Selecting rhododendrons

Rhododendrons are in the nurseries and home centers in the spring, when they are in bloom. This is the best time to make your selections. But this is not the best time to buy and plant new rhododendrons! Remember what I said about being subject to stress during the dry summer? That's tough on new transplants. Here in Zone 8 at least, the best time to buy and plant the selections you made in the spring is in the fall when the rains have returned. This helps relieve transplant stress and gives them a chance to begin to establish a root system before winter, and certainly before next summer. Most local nurseries won't have them then—people want to buy blooming plants and Rhododendrons bloom in spring. So you might have to get them from a grower, and might even get better plants that way. If you have to plant them in spring, make sure you water them especially well that first summer or two until they are well established.

The exceptions to that advice are the deciduous azaleas. Being dormant all winter makes them too susceptible to root rot. Plant them in early spring. Preferably just before they "bud-out".

Some hybrids are susceptible to root weevils, most relatively resistant. The leaf edges are notched, like someone with a hole-punch. Spraying with a systemic like Orthene works, because the critters are nocturnal.

The biggest mistake I think people make is to ignore the mature size of the variety they have selected in the gallon pot. Some varieties of Rhododendrons may never get knee-high, others will get the size of trees. Get a book or grower's catalog and check it out. I recommend Greer's Guidebook to Available Rhododendrons.

About specific choices, first know that there is no true blue in rhodys. To call Blue Jay or the like anything more than somewhat lavender is over-generous in my opinion. Likewise, the yellows lack brillance in the evergreens, look to the deciduous Exbury azaleas. There are good whites, but many whites have faint blushes of pink or yellow, especially in bud. Rhodys excel in pinks of all shades; light pinks, dark pinks, purply pinks, intense pinks. But we also have good, true reds—so don't settle for an intense pink if what you want is a red.

• Since I mentioned blues first, Ocean Lake, or Crater Lake, followed by Blaney's Blue are about the best I've seen. But they are about the color of blue vinca or scilla; bluish but not delphinium blue. There are some selected clones of the R. augustinii species, like chasmanthum, that are passable.
• Crest and Hotei are about the best clear yellow evergreens. But Crest is noted for discarding leaves after one year—keep it well pinched and branching so it has lots of terminals leafing. Hotei is notably susceptible to root rot, so make sure it has perfect drainage. There are other yellows, for example Nancy Evans, Lemon Tart, and Yellow Rolls Royce among others, that don't have these problems, but they might be a bit paler or have a little more blush of pink.
• Sappho, a purple splotched white, is noted for getting leggy, but this is because it is looking for light. There is one in my neighborhood planted exposed in full sun that is a perfectly tight and tidy. Normally rhodys are planted with some shade—a rule well violated with this hybrid in this area, Zone 6.
• Loderi Venus is worth mentioning. It's large and pink, but it is also wonderfully fragrant! (Apples, watermelon and pineapple.) Puget Sound is another fragrant pink. There isn't much in the way of fragrance in most rhodys, and what there is in the evergreens usually comes from the R. fortunei series of species and their hybrids. Unfortunately, there aren't many of these in the trade. There are some very fragrant native American deciduous azaleas from the east coast R. atlanticum species and hybrids, and west coast R. occidentale. But fragrant rhodys do exist if you're willing to hunt for them!
• Avalanche is one of my favorite rhodys for scent. The scent has lots of vanilla in it, with a dash of cinnamon and nutmeg—it reminds me of the kitchen when Mom's baking cookies! The flower is pale pink with red in the throat. The one I'm most familiar with is about 10' tall, but it's an old specimen plant.
• Taurus has to be one of my favorites! It's red. And it isn't suitable for a foundation planting—it's huge! I know examples of both Taurus and Loderi Venus that are well over 9' tall. But if you've got the proper place, a Taurus will make an unforgettable show in spring! As you can see in the picture above, Vulcan is also "Coke can" red.
• Mrs. Furnival and Furnival's Daughter are also favorites. They are pinks with red blotches. Perhaps they are favorites because they remind me of strawberry ice cream!
• Rainbow is pretty with a white throat and dark pink edging on the petals. President Roosevelt adds uniquely varigated leaves to similarly colored flowers.
• Rhododendrons may not do well in the south and south central states. But there are "east coast" varieties that will survive down to –20°F or so, and are suitable for most of the country. Some names are: Catalgla, white, –25°F; Nova Zembla, red, –25°F; Mrs Furnival, pink with red blotch, –15°F; Anah Kruschke, reddish purple, –15°F; PJM, light purple, –25°F; Exbury Azaleas, yellows and oranges, –25°F.